5. Air cut
The first devices to be used were atmo-spheric pressure air circuit breakers (magnetic circuit breakers).
Despite its relatively low dielectric strength and high de-ionization time constant (10 µs), air at atmospheric pressure can be used for breaking up to voltages of around 20 kV. This requires sufficient cooling power and a high arc voltage after current zero crossing to avoid thermal runaway.
5.1 Air cut-off mechanism
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The principle adopted is to keep the arc short enough, as long as the current is high, to limit the energy dissipated, then to lengthen it only as the current approaches zero.
This principle has led to the creation of a cut-off chamber for each device pole. This is a...
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Air cut
Oil cut
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